RNA interference (RNAi) is an evolutionarily conserved process in which recognition of double-stranded RNA (dsRNA) ultimately leads to posttranscriptional suppression of gene expression. This suppression is mediated by short dsRNA, also called small interfering RNA (siRNA), which induces specific degradation of mRNA through complementary base pairing. In several model systems, this natural response has been developed into a powerful tool for the investigation of gene function (see, e.g., Elbashir et al., Genes Dev., 15:188-200 (2001); Hammond et al., Nat. Rev. Genet., 2:110-119 (2001)). More recently, it was discovered that introducing synthetic 21-nucleotide dsRNA duplexes into mammalian cells could efficiently silence gene expression.
Although the precise mechanism is still unclear, RNAi provides a potential new approach to downregulate or silence the transcription and translation of a gene of interest. For example, it is desirable to modulate (e.g., reduce) the expression of certain genes for the treatment of neoplastic disorders such as cancer. It is also desirable to silence the expression of genes associated with liver diseases and disorders such as hepatitis. It is further desirable to reduce the expression of certain genes for the treatment of atherosclerosis and its manifestations, e.g., hypercholesterolemia, myocardial infarction, and thrombosis.
A safe and effective nucleic acid delivery system is required for RNAi to be therapeutically useful. Viral vectors are relatively efficient gene delivery systems, but suffer from a variety of limitations, such as the potential for reversion to the wild-type as well as immune response concerns. As a result, nonviral gene delivery systems are receiving increasing attention (Worgall et al., Human Gene Therapy, 8:37 (1997); Peeters et al., Human Gene Therapy, 7:1693 (1996); Yei et al., Gene Therapy, 1:192 (1994); Hope et al., Molecular Membrane Biology, 15:1 (1998)). Furthermore, viral systems are rapidly cleared from the circulation, limiting transfection to “first-pass” organs such as the lungs, liver, and spleen. In addition, these systems induce immune responses that compromise delivery with subsequent injections.
Plasmid DNA-cationic liposome complexes are currently the most commonly employed nonviral gene delivery vehicles (Felgner, Scientific American, 276:102 (1997); Chonn et al., Current Opinion in Biotechnology, 6:698 (1995)). For instance, cationic liposome complexes made of an amphipathic compound, a neutral lipid, and a detergent for transfecting insect cells are disclosed in U.S. Pat. No. 6,458,382. Cationic liposome complexes are also disclosed in U.S. Patent Publication No. 20030073640.
Cationic liposome complexes are large, poorly defined systems that are not suited for systemic applications and can elicit considerable toxic side effects (Harrison et al., Biotechniques, 19:816 (1995); Li et al., The Gene, 4:891 (1997); Tam et al, Gene Ther., 7:1867 (2000)). As large, positively charged aggregates, lipoplexes are rapidly cleared when administered in vivo, with highest expression levels observed in first-pass organs, particularly the lungs (Huang et al., Nature Biotechnology, 15:620 (1997); Templeton et al., Nature Biotechnology, 15:647 (1997); Hofland et al., Pharmaceutical Research, 14:742 (1997)).
Other liposomal delivery systems include, for example, the use of reverse micelles, anionic liposomes, and polymer liposomes. Reverse micelles are disclosed in U.S. Pat. No. 6,429,200. Anionic liposomes are disclosed in U.S. Patent Publication No. 20030026831. Polymer liposomes that incorporate dextrin or glycerol-phosphocholine polymers are disclosed in U.S. Patent Publication Nos. 20020081736 and 20030082103, respectively.
A gene delivery system containing an encapsulated nucleic acid for systemic delivery should be small (i.e., less than about 100 nm diameter) and should remain intact in the circulation for an extended period of time in order to achieve delivery to affected tissues. This requires a highly stable, serum-resistant nucleic acid-containing particle that does not interact with cells and other components of the vascular compartment. The particle should also readily interact with target cells at a disease site in order to facilitate intracellular delivery of a desired nucleic acid.
Recent work has shown that nucleic acids can be encapsulated in small (e.g., about 70 nm diameter) “stabilized plasmid-lipid particles” (SPLP) that consist of a single plasmid encapsulated within a bilayer lipid vesicle (Wheeler et al., Gene Therapy, 6:271 (1999)). These SPLPs typically contain the “fusogenic” lipid dioleoylphosphatidylethanolamine (DOPE), low levels of cationic lipid, and are stabilized in aqueous media by the presence of a poly(ethylene glycol) (PEG) coating. SPLPs have systemic application as they exhibit extended circulation lifetimes following intravenous (i.v.) injection, accumulate preferentially at distal tumor sites due to the enhanced vascular permeability in such regions, and can mediate transgene expression at these tumor sites. The levels of transgene expression observed at the tumor site following i.v. injection of SPLPs containing the luciferase marker gene are superior to the levels that can be achieved employing plasmid DNA-cationic liposome complexes (lipoplexes) or naked DNA.
Thus, there remains a strong need in the art for novel and more efficient methods and compositions for introducing nucleic acids such as siRNA into cells. In addition, there is a need in the art for methods of downregulating the expression of genes of interest to treat or prevent diseases and disorders such as cancer and atherosclerosis. The present invention addresses these and other needs.